Injuries of the head and neck are among the most devastating suffered by human beings. Despite great advances in safety equipment, they remain a leading cause of death and disability in our society.
Since many, if not most of these injuries occur during adolescence and young adulthood, such events may be considered even more costly to society in terms of productivity lost and medical costs endured. Indeed, a recent article in The Journal of the American Medical Association cites the great cost to society of motorcycle injuries alone.
Improvements in the ability of protective headgear to insulate the skull and its contents from trauma seem to have reached a plateau. Changes in helmet design are now oriented more toward comfort and weight savings. While these changes seem appropriate, typically any blow severe enough to overwhelm a modern helmet's defense would probably produce devastating damage to the neck, or cervical spine. Therefore, a system to protect the cervical spine is the next step in the evolution of safety equipment. Only when a practical system for protecting the cervical spine is achieved will further improvement of head protection be worthwhile.
The cervical portion of the spine is somewhat unique in that it lacks the extensive supporting musculature of the rest of the vertebral column. As the skull is carried close to its center, and hence, supported against the pull of gravity, neck muscles are mainly designed to facilitate movement. The principle neck muscles are the sternomastoid, which flex and rotate the head, and the trapezius, which extends it.
FIG. 1 is an exploded view of human cervical vertebrae, comprising: posterior tubercle 10; groove for vertebral 11; anterior arch facet for dens 12; interior tubercle 13; inferior articular process 14; transverse process 15; superior articular facet 15; dens (odontoid process) 17; transverse process posterior tubercle 18; costo-transverse bar 19; anterior tubercle 20; path of the vertebral artery (blood supply) 21; foramen transversarium 22; spine 23; lip 24; articular process inferior 25 and superior 26; carotoid tubercle 27; vestigial anterior tubercle 28; body 29; lateral mass tubercle for transverse ligament 30; superior articular process 31; atlas 34; axis 35; third cervical vertebrae 36; fourth cervical vertebrae 37; fifth cervical vertebrae 38; sixth cervical vertebrae 39; and seventh cervical vertebrae 40.
FIG. 2 is a front view of human articulated cervical vertebrae. FIG. 2 comprises many of the elements of FIG. 1, and further includes transverse process anterior tubercle 41, and gutter for nerve 42. FIG. 3 is a side view of human articulated cervical vertebrae. FIG. 3 includes many of the elements of FIGS. 1 and 2, and further comprises: vertebral artery 43; spinous processes or spines 44; column of articular processes 45; and lamina 46.
FIG. 4 is a cut-away side view of the intervertebral disc and ligaments in humans. FIG. 4 includes some of the elements in FIGS. 1-3, and further comprises: anterior longitudinal ligament of the bodies of the vertebrae 47; posterior longitudinal ligament of the bodies of the vertebrae 48; ligamentum flavum 49; interspinous ligament 50; supraspinous ligament 51; bursa 52; nucleus pulposus 53; intervertebral discs 54; cavity for nucleus pulposus 55; annulus fibrosus 56; hyaline plate 57; nucleus pulposus protruding into bodies 58; canal for basi-vertebral vein 59; ventral and dorsal nerve roots 60; and dura mater 61.
Referring to FIGS. 1-4 cervical vertebrae can be visualized as two short, adjoining cylinders, the larger of which is the vertebral body. This is the load-bearing structure of the spinal column. Cervical vertebrae are solid and separated from adjacent members by resilient fibrocartilaginous structures called intervertebral discs.
The spinal cord is carried in the adjacent, hollow cylinder formed by the laminae (arch) in a space known as the spinal foramen. The blood supply to the spinal cord, the vertebral artery, is carried in holes through bony projection lateral to the vertebral bodies. Additionally, there is a bony projection posteriorly from the arch called the spinous process. Ligaments connect these and the other structures, contributing to the strength of this system.
The areas of the spinal column most often affected by injury are the fourth and fifth cervical vertebrae, and the eleventh and twelfth thoracic. In the latter, the intrinsic strength of muscle groups in the area provide considerable support. The cervical spine, as previously noted, has little muscle support. Therefore, the considerable mass of the head acts as a pendulum or dead weight during impact to or sudden movement of the body.
In instances of spinal cord trauma, rapid and excessive movement and/or compression of the cervical spine occurs, tearing intervertebral ligaments, compressing and rupturing discs and vertebral bodies. The spinal cord, trapped in the spinal foramen, may be compressed by bone fragments or an extruded disc, or it may be stretched, interrupting its blood supply or tearing its nerves. In a series of cervical spine injuries studied by Bohlman and Boada, described in their work Fractures and Dislocations of the Lower Cervical Spine, one third of such injuries were due to motor vehicle accidents; their incidence is highest in adolescents and young adults.
Disc injuries were found to be most common. Brain injuries associated with spinal cord lesions in 61% of cases and the spinal cord lesions with brain injuries in 63% of cases, indicate the close association of cord injuries with head trauma. In very few cases was total spinal cord disruption noted. Instead, significant nerve damage was found to be primarily due to ischemia (interruption of the blood supply), and was improved most by early stabilization and reduction.
Immobilization should be carried out as soon as possible after a cervical spine injury is recognized since continuous movement may accentuate the pathologic processes that are already underway within the spinal cord as a result of the injury. Thus, often a soft collar with spinal traction is recommended as soon as possible after trauma.
The most common forces causing spinal cord injury are:
1) Flexion
2) Flexion rotation
3) Vertical (axial) loading with slight flexion
4) Extension.
1) Flexion
Straight flexion injury is by far, the most common injury in the cervical spine, often together with crumbling of a large portion of the superior anterior portion of the lower vertebrae, and also involving tearing of the ligaments between vertebral processes and stretching the spinal cord, as shown in FIGS. 5 and 6. There is interruption of the blood supply to the tissues, microscopic hemorrhage, and swelling. Since the swelling occurs in a confined space, increased pressure further impairs the blood supply and further tissue damage ensues.
2) Flexion rotation
The head is turned at the time of flexion resulting in unilateral ligamentous and bone injury and tissue injury similar to those in pure flexion.
3) Vertical or axial loading with slight flexion
In this instance, with reference to FIGS. 7-11, the vertebral body may be crushed and squeezed into the spinal foramen. This causes damage to the spinal cord both by direct pressure and indirectly by impairing its blood supply. The vertebral disc may be extruded into the foramen with similar results.
FIG. 11 shows an interior wedge fracture. In FIG. 8, crushing of the whole body is shown. FIG. 9 shows a posterior fragment of the vertebrae pushing out against the spinal cord. Finally, in FIG. 10, final displacement with a crushed body in flexion with posterior displacement of the vertebral body fragments is shown.
4) Extension
In cervical cord hyperextension injury 62, intervertebral ligaments and discs are torn as shown in FIG. 12. Spinous processes are jammed together and fractured at the base, decreasing the cervical spine's resistance to flexion injury and spinal cord 63 stretching as the head rotates forward in reaction. Flexion-extension injuries are the type most commonly occurring in automobile accidents.
Plainly, some sort of support system used in conjunction with a helmet is required to prevent or lessen the severity of these injuries. Currently, the only devices available are of the fixed type, usually consisting of a fabric-covered resilient foam collar between the helmet and shoulders. This design has gained wide acceptance in automotive racing but has some distinct drawbacks, as discussed elsewhere in this specification.
Note that there is virtually no use of such devices among motorcyclists and pilots. The restriction of head mobility that "collars" produce is typically unacceptable to them. Unfortunately, however, such people are at significant risk for spinal cord trauma. Even though a number of safety and protective devices are provided for pilots of military and private aircraft, there is no system for restricting head mobility at impact of a crash or during ejection from disabled aircraft.
Other disadvantages with fixed devices go beyond their limited acceptance. Rotational and flexion-extension injuries are the most prevalent in auto accidents. Current designs do not provide significant protection against extreme flexion, and the limited areas of contact with that helmet may actually provide a fulcrum, raising the center of rotation and increasing traction forces on the spinal cord and exacerbating injury.
The prior art includes U.S. Pat. No. 3,900,896, which relates to a neck brace for athletes, such as football players, for protecting the athlete from possible neck fractures or spinal cord injuries. The neck brace described generally comprises a rigid member vertically disposed immediately posterior and parallel to the neck of the athlete, with the upper end secured to the protective helmet and the lower end supported on a bracket constituting a part of the suit or shoulder pad of the athlete. While coupling of the rigid member and the lower bracket is described as providing for free rotation of the member around a vertical axis generally parallel to the neck, the amount of actual free rotation is uncertain. In addition, this invention unequivocally teaches limited forward and backward tilting of the head. Finally, to the obvious discomfort of the wearer, the neck brace must be used with a lower supporting member. The restriction of head mobility is simply unacceptable to most sports participants, including motorcyclists and drivers of other similar vehicles such as power boats, jet skis, snow mobiles and the like.
In U.S. Pat. No. 3,930,667, an inflatable garment for crash protection to be worn by a motorcycle rider is described. The garment is detachably connected to a source of pressurized gas operative to inflate the suit in response to a predetermined deceleration of the motorcycle or manual operation when a crash or spill appears inevitable. The source of pressurized gas is disposed on the motorcycle. Thus, if the rider jumps or is thrown from the motorcycle before the garment or suit is fully inflated, protection for the rider from the first or multiple impacts thereafter is compromised. Moreover, while the garment or suit described may be effective for protecting the back and spine, it appears to be ineffective for protecting the cervical portion of the spine or the head of the rider.
Finally, U.S. Pat. No. 4,825,469 also teaches motorcycle safety apparel, which in the event of an impending or actual accident will inflate to provide a protective enclosure for parts of the body most susceptible to critical or fatal injury. However, again the source of compressed or liquified gas is disposed on the motorcycle which compromises the overall effectiveness of the garment in the same way discussed with respect to U.S. Pat. No. 3,930,667. Several different embodiments of the safety apparel are described and typically include an inflatable hood which expands upward and then forward around the top and sides of the head. However, it is uncertain that flexion and flexion rotation injuries are prevented upon impact, or that damage from axial loading or extension injuries are even reduced.
At any early stage in the development of the present invention, an article was published regarding the present invention. See, Thompson, Steven L., "Dr. Archer's Air Bag", Cycle World, February 1989 issue.